Conventionally, a method of producing a plastic spectacle lens has been conducted by polishing an unpolished spectacle lens which is generally called a semi-finished lens and has a thickness greater than a finished size, as described in Japanese Patent Laid-open No. sho 47-14776. The semi-finished lens is fixed to a holding means for polishing by use of a low melting point alloy generally called “alloy”, the surface on the eyeball side (concave surface side) of the semi-finished lens fixed to the holding means is roughly ground in a spherical surface or a toric surface shape according to the prescription for the wearer, and is then subjected to sanding, which is similar to lapping, and to mirror surface polishing, whereby the optical surface of the lens is finished in precision.
In the sanding and polishing, a hard polishing dish made of aluminum, a hard plastic, foamed polyurethane or the like and having a curved surface shape is used. In addition, according to the purpose of processing, a polishing pad of a different kind is adhered to the surface of the hard polishing dish, and sanding and polishing are conducted. In concrete, the purpose of the sanding is to smoothen the roughly ground surface to a certain degree of surface roughness, and to grind off shape errors generated upon rough grinding so as to conform to the shape of the hard polishing dish. The sanding is conducted by a method in which a polishing pad for sanding called “fining pad” and holding on the surface thereof abrasive grains of silicon dioxide, aluminum oxide, silicon carbide or the like about 10 to 20 μm in grain diameter is adhered to the surface of the polishing dish, and sanding is conducted while pouring water. Though depending on the roughly ground condition, the sanding is generally conducted in two or more stages until a surface roughness in terms of Rmax of equal to or less than 10 μm is finally obtained.
The purpose of the subsequent mirror surface polishing is to further smoothen the sanded surface and to finish to a desired optical surface. The polishing is conducted by a method in which a polishing pad for polishing formed of nonwoven fabric is adhered to the surface of the polishing dish used in the sanding or a polishing dish of the same shape as the polishing dish used in the sanding, and polishing is conducted while supplying a slurry including abrasive grains dispersed in a dielectric liquid.
FIG. 5 shows a mirror surface polishing method using a polishing dish. FIG. 5(a) is a general side view showing the condition where a concave surface of a lens is polishing by use of the polishing dish, and FIG. 5(b) is a top plan view of the polishing dish. As shown in FIG. 5(b), a petal-shaped polishing pad 100 having a petal shape is adhered to the surface of a dome-shaped polishing dish 110 by utilizing an adhesive provided on the back side of the polishing pad 100. Most of polishing pads 100 commercially available for polishing a plastic spectacle lens each have a petal shape comprising a circular pad provided with radial notches 101 so as to conform to the curved surface shape of the dome-shaped polishing dish 110. As shown in a sectional view of FIG. 6, the commercially available polishing pad 100 has a structure in which a low-tack adhesive layer 105 is provided on one side of a nonwoven fabric 103, with a primer layer 104 therebetween. The polishing pad 100 is adhered to the polishing dish 110 by utilizing the low-tack adhesive layer 105. The polishing pad 100 covers substantially the surface of the polishing dish 110. As shown in FIG. 5(a), polishing is conducted by fixing the convex surface of a lens 8 as a work to a work fitting portion 10 through a joining member 9 such as a low melting point alloy, pressing the polishing dish 110 against the concave surface of the lens 8, rotating the polishing dish 110, and rotating and oscillating the lens 8 through the work fitting portion 10.
In recent years, however, high value added lenses in which the surface on the eyeball side of a spectacle lens is provided with an axially symmetric non-spherical surface, a non-spherical astigmatism surface, a progressive surface, a composed surface of a toric surface and a progressive surface, or the like have come to be produced. In the cases of these so-called free curved surface shapes, it is impossible to continuously rub the entire surface of the lens by use of a solid polishing dish having a convex surface in the same shape as the concave surface of the lens.
Therefore, the polishing dish 110 shown in FIG. 5 is changed to an elastic base member having a curved surface shape, the polishing pad 100 with the petal shape shown in FIG. 5 is adhered to the elastic base member to form an elastic polisher, the elastic polisher and the lens are rubbed against each other while pressing the elastic polisher against the concave surface of the lens, and the above-mentioned slurry is intermediately provided, whereby mirror surface finishing is performed. Since the elastic base member can be rubbed against the lens while deforming following up to the concave surface shape, it is possible to polish a free curved surface. As an example of the method of polishing following up to the work shape in this manner, there may be mentioned the method disclosed in Japanese Patent Laid-open No. Hei 8-206952.
In the case of using such an elastic polisher, the size of the elastic base member 2 is much larger than that of the polishing pad 100 which is roughly equal to the lens 8 in diameter, as shown in FIG. 7. The reason is that, since the polishing speed of the elastic polisher is lower than that of the hard polishing dish, it is necessary to enhance the circumferential speed so as to enhance the polishing speed. Therefore, the outside diameter of the polishing pad 100 is much smaller than that of the elastic base member 2, and, in order to cover the moving region 6 of the work 8, the polishing pad 100 is adhered in the state of being eccentric with respect to the center of the elastic base member 2, as shown in FIG. 7.
However, a polishing method using an elastic polisher obtained by adhering a conventionally commercialized polishing pad 100 for polishing a plastic spectacle lens to the surface of the elastic base member 2 has the following problems:
The polishing pad 100 is generally of the so-called disposable type such that the polishing pad 100 is replaced with a new one each time one work is processed. In addition, since the polishing pad 100 is presumed to be used by being adhered to the hard polishing dish, the adhesive force is set to be low taking into account the workability at the time of stripping. Namely, it suffices for the adhesive force to endure at least one polishing step. Therefore, where this type of polishing pad 100 is adhered to the elastic base member 2, the lowness of the adhesive force leads to the following problems.
First, as shown in FIG. 7, at the time of polishing by relative movements of the lens 8 and the elastic base member 2 while pressing the elastic base member 2 against the lens 8 at a predetermined polishing pressure, the elastic base member 2 is deformed by being pushed by the circumferential edge of the lens 8, so that the sharp circumferential edge of the lens 8 may easily get under the notch portion 101 or end edges of the polishing pad 100 having the petal shape. Therefore, the circumferential edge of the lens 8 will easily get between the polishing pad 100 and the elastic base member 2, to strip the polishing pad 100. Besides, since the adhesive force is low, the polishing pad 100 cannot follow up to the deformation in shape of the elastic base member 2, whereby the polishing pad 100 is stripped in many cases.
When even a part of the polishing pad 100 is stripped, the polishing pad in the stripped part is folded, resulting in that the polished surface of the optical component part is scratched, a desired appearance quality cannot be obtained in most cases, and defective polish is generated. Therefore, it is difficult to polish stably under the condition where durability to polishing is thus low.
Further, in order to regenerate the polishing pad which is partially stripped from the elastic base member during polishing, it is necessary to re-adhere the polishing pad and the elastic base member with an adhesive.
The present invention has been made in consideration of the above-mentioned situations. Accordingly, it is an object of the present invention to provide a polisher such that a polishing pad adhered to an elastic base member is not easily stripped during polishing and defective polish is not generated.
It is also an object of the present invention to provide a polishing method using a polisher such that a polishing pad adhered to an elastic base member is not easily stripped during polishing and defective polish is not generated.